1. Field of the Invention
The invention relates to a MOS semiconductor device and a manufacturing method thereof, more particularly, to a MOS semiconductor device in which work functions of gate electrodes are adjusted to different values for MOS transistors having different conductivity types in a dual-gate structure using a metal as a gate electrode and a manufacturing method thereof.
2. Description of the Related Art
In a miniaturized CMOS semiconductor device, it is general to use a so-called dual-gate structure. In the dual-gate structure, a polysilicon highly doped with an n-type impurity (dopant) (hereinafter referred to as n+ polysilicon), for example, phosphorus, is used to a gate electrode in an n-type MOS semiconductor device. Further, a polysilicon highly doped with a p-type dopant (hereinafter referred to as p+ polysilicon), for example, boron, is used to a gate electrode in a p-type MOS semiconductor device. This is based on characteristics that the work function of polysilicon can be controlled by type and amount of dopant. In a designing of a surface-channel-type MOS semiconductor device, the dual-gate structure using polysilicon is widely used to attain an acceptable trade-off relationship with respect to major design factors such as the thickness of gate insulators and the substrate doping concentration.
However, the influence of the depletion layer formed in the polysilicon gate electrode has caused a more serious problem as devices have become further miniaturized in recent years. As described above, since the polysilicon electrode is doped with dopant with a high impurity concentration, the depletion layer in the polysilicon is extremely thin. However, in a next-generation device, in which the gate insulator thickness is as thin as 2 nm or less in equivalent thickness of a silicon oxide film (SiO2), the depletion layer causes an increase in the capacitance corresponding to several 10% or more of the effective gate insulator thickness. Further, enhancement of the performance by a reduction in the gate insulator thickness may be hindered.
In order to avoid the above problems, studies have been conducted to use metals as the gate electrodes. The metal used to a gate electrode is required to match the MOS semiconductor device manufacturing process. For example, it is required the following characteristics to be attained: a metal film can be easily deposited; an etching process can be easily performed to form gate electrodes; an etching selectivity to the gate insulator is high; and the gate electrode is thermally stable against the gate insulator in heat treatments after forming the gate electrode. Since the work functions of the metals are different depending on respective materials and have distributed widely, it is possible to form gate electrodes having work functions equivalent to those of n+ and p+ polysilicon by selecting two kinds of adequate metals. For example, the use of titanium (Ti) to a gate electrode of an n-type MOS semiconductor device and of molybdenum (Mo) to a gate electrode of a p-type MOS semiconductor device has been reported by Q. Lu et al. in “2000 Symposium on VLSI Technology Digest of Technical Papers”, pp. 72-73. However, when this technique is applied to a CMOS semiconductor device production in practice, the manufacturing process becomes too complicated and is not suitable for mass production.
It has been observed that Mo is thermally stable in the heat treatment usually employed in the latest manufacturing process of a MOS semiconductor device even when Mo is formed on an insulator, such as SiO2 film, silicon nitride film (SiN) and hafnium oxide film (HfO2), which is used or may be used as a gate insulator. Therefore, Mo is expected to be used as a gate electrode material of future CMOS semiconductor devices. Further, a technique for implanting nitrogen ions into Mo to adjust the work function is reported by P. Ranade et al. in Mat. Res. Soc. Symp., Vol. 611, 2000, pp. C3.2.1-C3.2.6. However, it has been pointed out problems by the inventors and others based on their studies. That is, this method is not practical because of problems that the gate insulator and an interface between the gate electrode and the gate insulator are damaged in the implantation.
As another method for adjusting the work function of the Mo gate electrode, a method for thermally diffusing nitrogen from a titanium nitride (TiN) film deposited on Mo to underlying Mo has been reported by R. J. P. Lander et al. in Mat. Res. Soc. Symp. Proc. Vol. 716, 2002, pp. B5.11.1-B5.11.6. However, no study of a method for applying the technique to a CMOS semiconductor device with gate electrodes adjusting to have different work function values on the same semiconductor substrate has been reported.
Therefore, in order to realize a miniaturized CMOS semiconductor device, such as a device having a feature size of the 65 nm or less and having gate electrodes whose work functions are adjusted to different respective values, the following criteria must be met: (a) the gate electrodes of MOS semiconductor devices of different electrical conductivity types must be adjusted to have different work function values, respectively, (b) no depletion layer can be formed in the gate electrode in the operation of the MOS semiconductor device, (c) the manufacturing process must be simple and practicable, and (d) side effects must not occur in a MOS semiconductor device and/or a semiconductor manufacturing process, for example, damage to the gate insulator must be negligible.